A metal powder having an electric conductivity such as Ni, Cu, Ag or the like is used as a material forming an internal electrode for multilayered ceramic capacitor, a porous electrode for nickel-hydrogen secondary cell, a hollow porous electrode for fuel cell, an electrode for other electron parts, or the like. Particularly, metal super-micro powder having a particle size of not more than 0.4 μm is good in the paste nature and is possible to form a fine pattern of a conductor portion or thin the thickness thereof, so that it is rapidly increasing as a demand for the formation of conductors in an electron circuit.
As a production method of the metal super-micro powder are known an evaporation process in gas, an evaporation process in plasma, a vapor phase chemical reaction process, a reduction precipitation process in liquid phase and the like, and the development of these processes is proceeding in accordance with characteristics of particles, kind of metal, production scale and the like. Among them, the vapor phase chemical reaction process does not need an expensive apparatus and is considered to be advantageous in the cost and used in the production of the industrial scale.
The vapor phase chemical reaction process is a process wherein a metal compound having a low boiling point is evaporated to cause a thermal decomposition reaction or a reduction reaction to thereby precipitate particles from a vapor phase. In the production of the metal super-micro powder, a so-called vapor phase reduction method in which steam of a metal chloride is reduced by using a hydrogen gas is mainly applied. Because, the metal chloride is low in the boiling point and evaporable and is easily reduced to a metal by a reducing gas.
As the metal chloride constituting a starting material for the super-micro powder, a compound having a low valence is usually used even if the starting cost is somewhat high. For example, in case of producing Cu super-micro powder, CuCl is used as a starting material and subjected to a reduction reaction with hydrogen gas according to the following equation:2CuCl+H2→2Cu+2HClto produce super-micro powder. When CuCl2 having a valence larger than CuCl is used as a starting material, the material cost is cheap, but the reduction reaction proceeds at two stages as shown below, so that the reaction is complicated and the control thereof is difficult and hydrogen amount required for the reduction is 2 times and there is a problem that the production cost rather increases.2CuCl2+H2→2CuCl+2HCl2CuCl+H2→2Cu+2HCl
Therefore, if there is developed a technique capable of using a cheap and large valence. metal chloride instead of the expensive and low valence metal chloride as a starting material for the production of metal super-micro powder, it is possible to largely reduce the production cost.
Also, the vapor phase reduction process is utilized to not only the production of the metal single powder but also the production of alloy powder. As the method thereof are known
1) a method wherein metal chlorides as a target alloy component are previously mixed and the resulting mixture is heated to generate a mixed steam of the metal chlorides and reduced with a hydrogen gas;
2) a method wherein steams of metal chlorides as an alloy component are separately generated and these steams are introduced and mixed into a reaction tube and then reduced with a hydrogen gas, and the like. However, these methods have drawbacks that the cost of the starting material is high because the metal chloride having a low valence is used as the starting material but also the composition of the resulting alloy micro powder is not stable, and hence they have a problem in the practical use.
It is an object of the invention to propose a method of producing super-micro powder of a pure metal in which a cheap starting material can be used in a production method of metal super-micro powder through a vapor phase reduction process and powder can be efficiently produced. Also, it is another object of the invention to propose a cheap production method of alloy super-micro powder capable of cheaply and stably producing alloy super-micro powder having a desired composition.